Recent epidemiological and experimental animal data, as well as reanalyses of data previously accumulated, indicate that the lens of the eye is more radiosensitive than was previously thought. This has resulted in a reduction of the occupational lens dose limit within the European Union countries, Japan and elsewhere. This Commentary introduces the work done by the LDLensRad Consortium contained within this Focus Issue, towards advancement of understanding of the mechanisms of low dose radiation cataract.

Ionizing radiation is widely known to induce various kinds of lens cataracts, of which posterior subcapsular cataracts (PSCs) have the highest prevalence. Despite some studies regarding the epidemiology and biology of radiation-induced PSCs, the mechanism underscoring the formation of this type of lesions and their dose dependency remain uncertain. Within the current study, our team investigated the in vivo characteristics of PSCs in B6C3F1 mice (F1-hybrids of BL6 × C3H) that received 0.5–2 Gy γ-ray irradiation after postnatal day 70. For purposes of assessing lenticular damages, spectral domain optical coherence tomography was utilized, and the visual acuity of the mice was measured to analyze their levels of visual impairment, and histological sections were then prepared in to characterize in vivo phenotypes. Three varying in vivo phenotype anterior and posterior lesions were thus revealed and correlated with the applied doses to understand their marginal influence on the visual acuity of the studied mice. Histological data indicated no significantly increased odds ratios for PSCs below a dose of 1 Gy at the end of the observation time. Furthermore, our team demonstrated that when the frequencies of the posterior and anterior lesions were calculated at early time points, their responses were in accordance with a deterministic model, whereas at later time points, their responses were better described via a stochastic model. The current study will aid in honing the current understanding of radiation-induced cataract formation and contributes greatly to addressing the fundamental questions of lens dose response within the field of radiation biology.

One harmful long-term effect of ionizing radiation is cataract development. Recent studies have been focused on elucidating the mechanistic pathways involved in this pathogenesis. Since accumulating evidence has established a role of microRNAs in ocular diseases, including cataract, the goal of this work was to determine the microRNA signature of the mouse lens, at short time periods postirradiation, to understand the mechanisms related to radio-induced cataractogenesis. To evaluate the differences in the microRNA profiles, 10-week-old Patched1 heterozygous (Ptch1^+/–) mice, bred onto two different genetic backgrounds (CD1 and C57Bl/6J), received whole-body 2 Gy γ-ray irradiation, and 24 h later lenses were collected. Next-generation sequencing and bioinformatics analysis revealed that genetic background markedly influenced the list of the deregulated microRNAs and the mainly predicted perturbed biological functions of 2 Gy irradiated Ptch1^+/– mouse lenses. We identified a subset of microRNAs with a contra-regulated expression between strains, with a key role in regulating Toll-like receptor (TLR)-signaling pathways. Furthermore, a detailed analysis of miRNome data showed a completely different DNA damage response in mouse lenses 24 h postirradiation, mainly mediated by a marked upregulation of p53 signaling in Ptch1^+/–/C57Bl/6J lenses that was not detected on a CD1 background. We propose a strict interplay between p53 and TLR signaling in Ptch1^+/–/C57Bl/6J lenses shortly after irradiation that could explain both the resistance of this strain to developing lens opacities and the susceptibility of CD1 background to radiation-induced cataractogenesis through activation of epithelial-mesenchymal transition.

Epidemiological studies suggest an increased incidence and risk of cataract after low-dose (<2 Gy) ionizing radiation exposures. However, the biological mechanism(s) of this process are not fully understood. DNA damage and repair are thought to have a contributing role in radiation-induced cataractogenesis. Recently we have reported an inverse dose-rate effect, as well as the low-dose response, of DNA damage and repair in lens epithelial cells (LECs). Here, we present further initial findings from two mutated strains (Ercc2^+/– and Ptch1^+/–) of mice, both reportedly susceptible to radiation-induced cataract, and their DNA damage and repair response to low-dose and low-dose-rate gamma rays. Our results support the hypothesis that the lens epithelium responds differently to radiation than other tissues, with reported radiation susceptibility to DNA damage not necessarily translating to the LECs. Genetic predisposition and strain(s) of mice have a significant role in radiation-induced cataract susceptibility.

Experimental mouse studies are important to gain a comprehensive, quantitative and mechanistic understanding of the biological factors that modify individual risk of radiation-induced health effects, including age at exposure, dose, dose rate, organ/tissue specificity and genetic factors. In this study, neonatal Ptch1^+/– mice bred on CD1 and C57Bl/6 background received whole-body irradiation at postnatal day 2. This time point represents a critical phase in the development of the eye lens, cerebellum and dentate gyrus (DG), when they are also particularly susceptible to radiation effects. Irradiation was performed with γ rays (⁶⁰Co) at doses of 0.5, 1 and 2 Gy, delivered at 0.3 Gy/min or 0.063 Gy/min. Wild-type and mutant mice were monitored for survival, lens opacity, medulloblastoma (MB) and neurogenesis defects. We identified an inverse genetic background-driven relationship between the radiosensitivity to induction of lens opacity and MB and that to neurogenesis deficit in Ptch1^+/– mutants. In fact, high incidence of radiation-induced cataract and MB were observed in Ptch1^+/–/CD1 mutants that instead showed no consequence of radiation exposure on neurogenesis. On the contrary, no induction of radiogenic cataract and MB was reported in Ptch1^+/–/C57Bl/6 mice that were instead susceptible to induction of neurogenesis defects. Compared to Ptch1^+/–/CD1, the cerebellum of Ptch1^+/–/C57Bl/6 mice showed increased radiosensitivity to apoptosis, suggesting that differences in processing radiation-induced DNA damage may underlie the opposite strain-related radiosensitivity to cancer and non-cancer pathologies. Altogether, our results showed lack of dose-rate-related effects and marked influence of genetic background on the radiosensitivity of Ptch1^+/–mice, supporting a major contribution of individual sensitivity to radiation risk in the population.

Recent epidemiological findings and reanalysis of historical data suggest lens opacities resulting from ionizing radiation exposures are likely induced at lower doses than previously thought. These observations have led to ICRP recommendations for a reduction in the occupational dose limits for the eye lens, as well as subsequent implementation in EU member states. The EU CONCERT LDLensRad project was initiated to further understand the effects of ionizing radiation on the lens and identify the mechanism(s) involved in radiation-induced cataract, as well as the impact of dose and dose-rate. Here, we present the results of a long-term study of changes to lens opacity in male and female adult mice from a variety of different genetic (radiosensitive or radioresistant) backgrounds, including mutant strains Ercc2 and Ptch1, which were assumed to be susceptible to radiation-induced lens opacities. Mice received 0.5, 1 and 2 Gy ⁶⁰Co gamma-ray irradiation at dose rates of 0.063 and 0.3 Gy min^–1. Scheimpflug imaging was used to quantify lens opacification as an early indicator of cataract, with monthly observations taken postirradiation for an 18-month period in all strains apart from 129S2, which were observed for 12 months. Opacification of the lens was found to increase with time postirradiation (with age) for most mouse models, with ionizing radiation exposure increasing opacities further. Sex, dose, dose rate and genetic background were all found to be significant contributors to opacification; however, significant interactions were identified, which meant that the impact of these factors was strain dependent. Mean lens density increased with higher dose and dose rate in the presence of Ercc2 and Ptch1 mutations. This project was the first to focus on low (<1 Gy) dose, multiple dose rate, sex and strain effects in lens opacification, and clearly demonstrates the importance of these experimental factors in radiobiological investigations on the lens. The results provide insight into the effects of ionizing radiation on the lens as well as the need for further work in this area to underpin appropriate radiation protection legislation and guidance.

We have shown previously that a single radiation event (0.063, 0.125 or 0.5 Gy, 0.063 Gy/min) in adult mice (age 10 weeks) can have delayed dose-dependent effects on locomotor behavior 18 months postirradiation. The highest dose (0.5 Gy) reduced, whereas the lowest dose (0.063 Gy) increased locomotor activity at older age independent of sex or genotype. In the current study we investigated whether higher doses administered at a higher dose rate (0.5, 1 or 2 Gy, 0.3 Gy/min) at the same age (10 weeks) cause stronger or earlier effects on a range of behaviors, including locomotion, anxiety, sensorimotor and cognitive behavior. There were clear dose-dependent effects on spontaneous locomotor and exploratory activity, anxiety-related behavior, body weight and affiliative social behavior independent of sex or genotype of wild-type and Ercc2^S737P heterozygous mice on a mixed C57BL/6JG and C3HeB/FeJ background. In addition, smaller genotype- and dose-dependent radiation effects on working memory were evident in males, but not in females. The strongest dose-dependent radiation effects were present 4 months postirradiation, but only effects on affiliative social behaviors persisted until 12 months postirradiation. The observed radiation-induced behavioral changes were not related to alterations in the eye lens, as 4 months postirradiation anterior and posterior parts of the lens were still normal. Overall, we did not find any sensitizing effect of the mutation towards radiation effects in vivo.

Cataract is the leading cause of visual impairment which can result in blindness. Cataract formation has been associated with radiation exposure; however, the mechanistic understanding of this phenomenon is still lacking. The goal of this study was to investigate mechanisms of cataract induction in isolated lens epithelial cells (LEC) exposed to ionizing radiation. Human LECs from different genetic backgrounds (SV40 immortalized HLE-B3 and primary HLEC cells) were exposed to varying doses of ¹³⁷Cs gamma rays (0, 0.1, 0.25 and 0.5 Gy), at low (0.065 Gy/min) and higher (0.3 Gy/min) dose rates. Different assays were used to measure LEC response for, e.g., viability, oxidative stress, DNA damage studies, senescence and changes to telomere length/telomerase activity at two time points (1 h and 24 h, or 24 h and 15 days, depending on the type of assay and expected response time). The viability of cells decreased in a dose-dependent manner within 24 h of irradiation. Measurement of reactive oxygen species showed an increase at 1 h postirradiation, which was alleviated within 24 h. This was consistent with DNA damage results showing high DNA damage after 1 h postirradiation which reduced significantly (but not completely) within 24 h. Induction of senescence was also observed 15 days postirradiation, but this was not attributed to telomere erosion or telomerase activity reduction. Overall, these findings provide a mechanistic understanding of low-dose radiation-induced cataractogenesis which will ultimately help to inform judgements on the magnitude of risk and improve existing radiation protection procedures.

Lens epithelial cell proliferation and differentiation are naturally well regulated and controlled, a characteristic essential for lens structure, symmetry and function. The effect of ionizing radiation on lens epithelial cell proliferation has been demonstrated in previous studies at high acute doses, but the effect of dose and dose rate on proliferation has not yet been considered. In this work, mice received single acute doses of 0.5, 1 and 2 Gy of radiation, at dose rates of 0.063 and 0.3 Gy/min. Eye lenses were isolated postirradiation at 30 min up until 14 days and flat-mounted. Then, cell proliferation rates were determined using biomarker Ki67. As expected, radiation increased cell proliferation 2 and 24 h postirradiation transiently (undetectable 14 days postirradiation) and was dose dependent (changes were very significant at 2 Gy; P = 0.008). A dose-rate effect did not reach significance in this study (P = 0.054). However, dose rate and lens epithelial cell region showed significant interactions (P < 0.001). These observations further our mechanistic understanding of how the lens responds to radiation.